The field of the invention is reflex cameras and the invention is particularly concerned with improvements in the electronic rangefinders therefor. These rangefinders have a photoelectric detection system mounted behind a grating acting as a spatial frequency filter in a focusing plane of the image taking objective and the output signals from the detection system provide information about the degree of focus of the objective on the object being aimed at.
The state of the art of such reflex cameras may be ascertained by reference to U.S. Pat. Nos. 3,781,110; 3,953,728 and 4,188,102; the disclosures of which are incorporated herein.
An essential feature of these rangefinders as disclosed in U.S. Pat. No. 4,188,102 is a mirror ground on the lower edge of the pentaprism of the camera and the angles of the pentaprism are selected so that the viewfinder axis deviates downward by some angle with respect to the optical axis. For normal camera use the optical axis is horizontal. Moreover, the eyepiece is upwardly offset with respect to its position when the optical axis is horizontal and is slanted so that the eyepiece and viewfinder axes subtend an angle.
This inclination of the optical axis for the viewfinder sight on the one hand and the measuring beam on the other hand, however, suffers from the drawback that the angular changes then required at the pentaprism result in undesired reflections in the viewfinder image. Also, when objectives of long focal lengths and lesser light admission are used, the exit pupil of which is well known to be smaller than the eyepiece window, a bothersome darkening of the viewer image takes place. Again, it has been found that the mutual tilting of the two beams is required merely for the light flux passing through the "measurement spot" but not for the light flux from the object field.